Can't delete photos from my LG X Power

Updated with more information — please see below

I have an unrooted LG X Power running Android 6.0.1. I have recently gone through Google Photos (using a desktop web browser) and deleted a bunch of videos, but even though Photos tells me that they will be removed from my devices, they remain on my phone, and I can’t seem to get rid of them. When I try to delete them using the Photos app on the phone, I get the error message:

Can’t delete [x] items

Photos doesn’t have access to delete items on your SD card. Go to the Gallery app to continue deleting.

But I don’t even have a Gallery app on my phone! Moreover, when I use a file browser to see what’s on my SD card, there is no trace of them; they do not appear in the DCIM > Camera directory (although my photos do).

Where are these videos hiding? I suspect they are hidden somewhere in my internal storage, as I am rapidly running out of storage space, but the DCIM > Camera directory in my internal storage is completely empty, and I can’t seem to find any other large directory. Why doesn’t deleting them from Google Photos remove them from my phone? And why does Photos keep telling me to use a nonexistent Gallery app to delete them?

Edited to add: As far as I can tell, Photos does have permission to access the SD card; it says so in Preferences, and is able to save photos and videos there with no problem. It only throws up an error message when I try to delete. Furthermore, the error seems to affect photos and videos taken recently; older photos and videos (which were all taken on a different phone) can be deleted with no difficulty.

Further updates: Well, I found them! The files are not on my SD card, but rather are in my internal storage. But the plot thickens:

• Using the DiskUsage app by Ivan Volosyuk I was able to find the files hidden in my DCIM folder on my internal storage (which is odd, because Photos keeps telling me that it can’t delete them because it doesn’t have access privileges for my SD card… which it does… but anyway…). The folder, according to this app, contains about 5.9 GB of media. (There’s also a Podcasts folder with 1.33 GB that I’d like to get rid of, also…) See screenshot below, then keep reading for more of the mystery!

• When I try to delete the files (whether one at a time, or the whole folder at once) I get an error message: “Error: file wasn’t deleted.” No additional information about why the file wasn’t deleted is provided. See screenshots 2 and 3 below, then keep reading for more info!

• To further compound my frustrations, the files (and folders) in question don’t show up when I use other file browsers. Here, for example, is what I see in Android File Transfer (see my last screenshot below). Notice that the DCIM folder exists, but is essentially empty, except for a few thumbnails; the Podcasts folder isn’t shown at all. I get similar results when I use the stock File Manager app on my phone, and when I use this other File Manager app, and when I use Files By Google: none of these apps show the existence of the files in question at all. And yet they’re there, taking up space, visible in DiskUsage.

I’m hoping that with this additional information, someone can help me figure out how to finally delete these damn files. I could really use that 7 GB of space back.